The present invention relates to a method for preparing polycarbonates by a transesterification reaction between, for example, diaryl carbonate and aromatic bis hydroxy compounds. In particular, this invention relates to the melt polymerization reaction for the preparation of polycarbonates and novel polycarbonate catalysts comprised of certain cesium salts.
A large number of catalyst systems have been examined for application to the melt polymerization of polycarbonates. Most of these methods require either a variety of co-catalysts or the subsequent addition of a catalyst quencher to ensure polymer stability. The need for high purity, high quality thermoplastic resins requires the reduction of residual contaminants in the final resin. This need for very low residual impurities is particularly acute in optical quality (OQ) grade polycarbonate resins. One approach towards elimination of residual solvent contamination, particularly methylene chloride, is through the implementation of a solventless (i.e., melt) process.
The melt process generally involves a base catalyzed condensation polymerization of, for example, diphenyl carbonate, and a dihydroxy compound such as Bisphenol A. The reaction is conducted at high enough temperatures for the starting monomers and product to remain molten, while the reactor pressure is staged in order to effectively remove phenol, the by-product of the polycondensation reaction.
Most current melt technology programs employ a two component catalyst system. The first component is a tetralkylammonium hydroxide (TMAH ) co-catalyst which is used to initiate oligomer formation in the melt. The second catalyst is an alkali metal hydroxide (i.e., the xe2x80x9cxcex1-catalystxe2x80x9d) which is the second part of the overall catalyst system. Due to its intrinsic thermal stability, the alkali metal salt must be quenched at the end of the polymerization. This quenching process requires the addition of yet another component to the polymer formation. All materials from the quenching process remain in the final resin, further compromising the final properties.
Although the alkali metal hydroxides in general are excellent melt polymerization catalysts they tend to generate substantial amounts of an additional undesired by-product which is a branched polycarbonate species typically referred to as Fries product which has the repeat unit as set forth below. The formation of Fries product during melt polycarbonate polymerization leads to changes in ductility and in general rheological properties of the polymer. Polycarbonates produced by the melt process typically have higher Fries content than polycarbonates produced by the interfacial method. As used herein the term xe2x80x9cFriesxe2x80x9d or xe2x80x9cfriesxe2x80x9d refers to a repeating unit in polycarbonate having the following formula: 
wherein the X variable represents
wherein variables Rc and Rd each independently represent a hydrogen atom or a
monovalent hydrocarbon group and may form a ring structure. Thus, a need exists for the development of alternative melt polycarbonate polymerization catalysts which produce less Fries product than conventional catalyst systems.
This invention provides a method for preparing polycarbonates, which utilizes polycondensation catalysts of the formula Cs+Anxe2x88x92, wherein Anxe2x88x92represents anions such as CH3SO3xe2x88x92, NH2SO3xe2x88x92, C2H2O4xe2x88x92, BF4xe2x88x92, B (C6H5CH3)4xe2x88x92, and CH3C6H4SO3xe2x88x92. We have found that this new class of catalysts provides excellent polymerization rates for the preparation of Bisphenol A polycarbonate from the melt polymerization of diphenyl carbonate and Bisphenol A. Moreover, the catalysts of the invention were found to be very selective in substantially reducing the level of branching side reaction, i.e., formation of Fries product, normally associated with the melt polycarbonate process, while at the same time producing such polycarbonates at rates and molecular weights which are comparable to conventional NaOH-catalyzed reactions.